1. Field of the Invention
This invention relates generally to high pressure gas lasers which are repetitively pulsed and more particularly to the use of a uniform field electrode configuration utilizing cathode irradiation by corona discharge to provide initiatory electrons.
2. Description of the Prior Art
In a copending patent application Ser. No. 740,221, filed June 26, 1968, in the name of Coleman J. Miller and owned by the assignee of this application, there is described and claimed a high power stimulated emission of radiation device using transverse electric field excitation of a gaseous medium having molecules with vibrational and rotational energy levels, between two parallel electrodes capable of operating at atmospheric pressures and above. Prior to the invention in that application, there was the problem that the electrical discharge in the gaseous medium tended to occur in the form of an arc as the pressure in the laser device increased above some low value, typically 20 to 50 torr. Such an arc would be supported by a very limited portion of the gaseous medium in a small column around the arc and the gain consequently would not be sufficient to cause laser action. The result was localized heating of the gas generally preventing laser operation entirely.
In the Miller application, means for irradiating the cathode with ultraviolet light for producing free electrons by photoelectric emission from the cathode is disclosed. The free electrons in turn produce a glow discharge uniformly over the surface of the electrodes which is self-sustaining when the conditions of voltage and electrode gap dimensions are such that each electron leaving the cathode establishes secondary processes whereby it is replaced by a new electron leaving the cathode. The free electrons produced in the glow discharge near the cathode engage in exciting collisions with ions, atoms or molecules of the gaseous medium in regions more remote from the cathode. In these excitation regions there is a lower ratio of electric field to particle density E/N and an amplifying action results by the further interchange of energy between free electrons and unexcited particles and between excited and unexcited particles of the gas.
The geometry of the electrode configuration and the ratio of E/N are important in the above type laser apparatus to establish the proper conditions for transfer of energy from the electrical energy source to the gaseous medium. The choice of electrode geometry and E/N ratio can improve the efficiency of energy transfer from the electrical energy source into selected modes of excitation of the gaseous medium which in turn can increase the overall efficiency of the laser device for a selected output frequency.
In the above-mentioned Miller application, means in the form of an ultraviolet light source for irradiating the cathode is disclosed for producing free electrons by photoelectric emission from the cathode causing a uniform glow discharge over the surfaces of the electrodes. As previously noted the glow discharge is self-sustaining when the conditions of voltage and dimensions of the gap between the electrodes are such that each electron leaving the cathode establishes secondary processes whereby it is replaced by a new electron leaving the cathode. The free electrons in such a glow discharge are accelerated by the electric field and excite the ions, atoms or molecules of the gaseous medium in inelastic collisions in regions remote from the cathode thereby causing an amplifying action by way of the further exchange of energy between free electrons and unexcited particles and between excited and unexcited particles of the gas. Since there is an infinite number of points on the planar plate electrode, a substantially uniform diffused glow discharge can be maintained for a limited time between the plates so that energy may be transferred from the electric field to the molecules of the active gaseous medium. This makes it possible to operate a gas laser in a pulsed mode at pressures higher than what was generally considered the cut-off threshold pressure for such devices (in the neighborhood of 20 Torr).
Another prior art technique for providing free electrons to initiate the diffused glow discharge in a high pressure gas laser having parallel plate electrodes is to use auxiliary electrodes adjacent each main parallel plate electrode for discharge initiation. The auxiliary electrodes closest to the cathode initiate a trigger discharge between cathode and auxiliary electrodes to provide the initiatory electrons for the main gap which is pulsed from the same source. The auxiliary electrodes nearest the anode are said to "focus" the beam in the vicinity of the anode.
In another prior art device in which a plurality of pins comprising one electrode means is positioned opposite a bar electrode, triggering of the glow discharge is accomplished by field emission at the pins when an impulse voltage is applied. Initiatory electrons are thereby provided because of the inherent physical properties of the nonuniform field configuration.
In an analogous art area U.S. Pat. No. 2,990,492 issued to Wellinger et al teaches the application of a solid state radioactive medium to lightning arresters whereby the radioactive medium supplies free electrons to initiate a power absorbing arc stream or discharge for the purpose of protecting power lines. This patent is representative of the art dealing with those protective devices whose primary purpose is to develop an arc stream or discharge. Such devices are the antithesis of the present invention because it is the prevention of an arc stream or discharge which must be accomplished while producing a very fast rise time in a diffused glow discharge. An arc stream or discharge in a gas laser causes the device to cease operating immediately and can cause serious damage to the electrodes.
The present invention relates to the broad field of producing an improved transfer of energy from the electric field to the selected rotational vibrational modes of molecules of a gas medium in a laser system. Free electrons are generated and accelerated by the electric field applied to the gaseous medium. Since the energy that each electron receives from the electric field depends upon the strength of the field and the distance through which the electron is accelerated by the field between collisions, energy losses can be minimized in relation to the toal input energy to the electric field by adjusting certain parameters. Also, the transfer of energy from the electrons to the selected vibrational rotational modes of the gas molecules can be maximized.
Generally speaking, there are two basic systems that have been used in the prior art to which the present invention relates as far as electron excitation of the molecules is concerned. The gaseous medium can be excited by RF energy with electrodes placed external to the gaseous medium container or it can be excited by applying a voltage either DC or AC, across a pair of electrodes immersed in the gaseous medium. As a practical matter, the excitation process has a significant influence on the operation of a gas laser device. The optimum operating pressure is limited by thermal consideration. As the pressure increases above approximately 20 Torr in a static volume of gas there is a tendency for a discharge to occur in the form of an arc streamer. This creates heat with a very high thermal gradient which adversely affects the lasing operation. The objective in this art is then to create a self-sustaining diffused glow discharge and maintain it in this mode as long as possible before thermal effects cause an arc streamer. An arc streamer discharge will cause constriction of the discharge, rapid temperature rise, and immediate cessation of the lasing operation.